The Non-Techie’s Earth Station Primer
Or, Is That What That Dish-Shaped Thing Actually Does?
By James Careless
Admit it: most people can’t explain how a satellite earth station works in plain English, including many technicians who work on them for a living, and for whom terms like klystron, waveguide and Quadrature Phase Shift Keying (QPSK) actually make sense!
Well, as a public service, Via Satellite will now attempt to explain the ups and downs of earth stations in simple, clear words.
Wish us luck.
It all starts with a signal.
It could be a telephone call. Or a TV feed. Or even an Internet query.
But as far as a satellite uplink is concerned, it’s just a signal that’s got to go from ground to space.
Satellite uplink: that’s the short form for earth stations that transmit– “link”–data up to orbiting satellites.
To get the signal up there, however, there are a few things that have to happen.
It all starts with the “baseband.” These are the various frequencies that make up the signal itself.
In digital communication, the conversion from analog to digital occurs at this stage.
Sometimes this baseband signal and others like it are combined into a single transmission stream. The device that does this is called a “multiplexer.”
Next, the data stream is encrypted and encoded.
Encrypting prevents anyone without authorized equipment from viewing the stream: remember all the illegal C-band receivers of the ’70s? They’re no more!
Encoding allows something called “Forward Error Correction” (FEC) to be added. FEC adds unique data bits to the stream, which the receiver can then check to see if the signal’s coming through correctly or not. It’s akin to marking the transmission with little red marks. Any time the receiver gets a mark that’s not red, it knows that part of the transmission is garbled. Forward error correction actually fixes transmission errors. If it can’t be fixed then it flags the data for retransmission.
Next comes modulation. This means taking the baseband data stream and converting it to something that can be transmitted by varying one of the properties of the electromagnetic wave. (In digital signals, it’s the “phase” which is usually altered.) Modulation occurs at the Intermediate Frequency (IF) stage, where typically the center frequency is either 70 MHz or 140 MHz.
Why 70 MHz? “It’s traditional,” explains Howard Hausman, vice president of engineering at Miteq. “Seventy MHz is a Bell standard from 40 years ago, when satellites came into being. Meanwhile, 140 MHz is used for wideband signals that 70 MHz can’t accommodate.”
Now that our signal is modulated, it is “upconverted.” This means the IF signal is translated to the range of radio frequencies (RF) actually being used for transmission. For instance, for a C-band uplink, the stream is upconverted to 6 GHz. For Ku-band, it’s upconverted to 14 GHz.
(The high frequency and small wavelength used by the RF signal make it possible to use antennas with reasonable dimensions.)
Okay, it’s amplification time! After all, your average geosynchronous satellite (GEO; one that appears to stay in the same place in the sky, relative to the Earth’s rotation) is 22,300 miles away. Granted, LEO (low earth orbit) satellites are closer, but even they experience signal loss.
To do the job, you can use either tube or solid state amplifiers. Tube amplifiers come in klystron (narrow bandwidth; covers one transponder) or Travelling Wave Tube (TWT; covers 24 transponders). Meanwhile, solid state amplifiers (SSPAs) are just that: they’re not tubes.
For years, the satellite industry has debated the relative merits of tubes versus solid state. But we won’t, because this is a simple article, okay?
We’ll also leave alone the various forms of modulation, such as Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). This said, it’s easy to explain the difference between the two: namely that QPSK is twice as efficient as BPSK in using bandwidth, without needing any more power. Or, as Via Satellite Technical Editor (and Satellite Engineering Research Corp. President) Robert Nelson puts it, QPSK “is the digital communication equivalent of a free lunch.” He adds, “Work is now being done on 8PSK, which requires more power but only requires two-thirds of QPSK’s bandwidth to support the same data rate. Alternatively, for the same bandwidth, 8PSK lets you increase QPSK’s data rate by 50 percent.”
Back to business: we now have our amplified signal, just waiting to blast into space. We send it out through a waveguide–that’s a hollow tube that can transmit signals above 2 GHz, which coax and heliax cable can’t–to the satellite antenna.
We’ll be colloquial, and call it a “dish.”
To be precise, the signal goes to a feedhorn, which is mounted above the dish. If it’s a “direct feed” model, the feedhorn points down at the dish. The signals hit the dish: its curve focuses the signal into a distinct beam. If it’s an indirect feed, the horn points skyward to a subreflector, which then bounces the signal onto the main dish, and out into space.
Subreflectors are usually only used on large dishes, when signal quality is an issue. They reduce noise caused by ground heat, because the horn isn’t pointed at the Earth.
Interestingly, the signal beam doesn’t go up as a solid, uniform pattern. Instead, interference within the beam causes it to form “Airy Rings.” Named for the 19th century scientist who discovered them–and not because of the space between them–these rings are circles of strong signals, separated by dead zones. The secret here is to aim your dish so that the strong areas hit the satellite.
Did you do it? Congratulations! Your signal’s in space!
And Down Again
But what if you want to receive a signal from a satellite, rather than uplink one to it?
Well, all you have to do is reverse the process.
Specifically, the signal comes down from space, and is received by your earth station antenna.
Remember what I said about distance before? Well, thanks to the reduced power available from space-based transmitters–plus signal loss along the path–the signal you’re picking up is probably in the picowatt range. We’re talking 0.0000000000001 watts here, or 10-12.
In short, a tiny, weak, infinitesimal signal.
Equivalent to the audio output of a buzzing mosquito that you can just barely hear.
Depending on the incoming signal strength–plus the background interference at the receiving antenna–the signal may be picked up through the feedhorn, and then sent to a Low Noise Amplifier (LNA) or a Low-Noise Block Downconverter (LNB).
The LNA simply takes the signal, boosts it, and sends it back to the demodulator. The LNB does what the LNA does, then downconverts the signal to an IF in the L-band.
Which do you choose? Well, you only actually have a choice if you’re using C-band. For all higher frequencies like Ku- and Ka-band, LNBs are a must.
So it’s a balancing act. Each situation is different, depending on what bandwidth you’re working in.
Now it’s time to downconvert to 70 MHz, demodulate it again, then decode and de-encrypt. It’s also time for FEC, to verify that the data being received is the same as the data that was sent.
Finally, the decoded signals are routed to their next destinations: be it by fiber, coax, wireless, or even satellite uplink to other far-flung locations.
And that’s that!
Don’t Look Down
I say this, because when all is said and done, earth station engineering is like balancing on a technical tightrope.
If you use too much amplification, you waste power and money. If you use too little, your signal could get lost.
If you make your dish bigger, you can increase your signal’s strength. However, you also push up your equipment costs, because more dish means more material used, and more manufacturing required!
Then there’s noise. If you use an LNA, you may get more noise. If you use an LNB, you may also get more noise. You can also get noise if your waveform’s not working properly, if there’s leakage in your cables, if there’s an unshielded electrical source nearby, a kid with a walkie-talkie…you get the picture.
Then there are frequency choices. The higher you go the less crowded the bandwidth, and the better the antenna performance (which allows the use of smaller dishes, and thus less cost). That’s why Ku-band came in after C-band filled up. Ka-band is now taking over from Ku-band as it jams up, and so it goes.
The downside? The higher frequencies are less reliable, especially during rainstorms. The reason is wavelength size. The higher bandwidths use waves short enough to be either scattered or even absorbed by raindrops.
And That’s That!
When you get down to basics, satellite technology is pretty straightforward. You take the signal, convert it, boost it, aim it and send it. To receive from space, simply reverse the process.
Still, “the devil is in the details,” as the old saying goes. In the case of satellite earth stations, it’s the demons of interference–both natural and manmade–plus the physical limits of transmission media and energy, and the budgetary constraints all businesses face.
This said, there’s no reason why you can’t explain earth stations in simple English. If you still feel unsure, then clip this article out and keep it in your wallet. We won’t tell.
A Glossary Of Basic Earth Station Terms
Via Satellite thanks Quantum Prime Communications, a VSAT system installer based in Stafford, VA, for letting us use their online glossary for compiling this section.
Baseband: Signals in their original, unmodulated state.
BPSK (Binary Phase Shift Keying): A digital modulation scheme used in transmission communication
C-band: The band of frequencies used for satellite and terrestrial communications. The range of frequencies from 4 to 6 GHz (billion cycles per second) is used by most communications satellites. The 3.7 to 4.2 GHz satellite communication band is used for downlink frequencies in tandem with the 5.925 to 6.425 GHz band, which serves as the uplink.
Decoding, De-encryption, and Demodulation: The process of restoring a received satellite signal to its original unprocessed state by decoding, de-encrypting and demodulating it.
Downlink: A satellite receive system that processes satellite-delivered information. Functionally, it includes the satellite itself, the receiving earth station and the signal transmitted downward between the two.
Encoding and Encryption: Coding or otherwise scrambling transmission content, making it unusable or unseeable to viewers who do not have the specified decoding equipment. As well, Forward Error Correction codes are added to signal uplink streams at this stage.
Feedhorn: The receiving antenna component that collects the signal reflected from the main surface reflector (“the dish”) and channels this signal into the low-noise amplifier (LNA).
Forward Error Correction (FEC): An error-correction technique that uses redundant information passed with the actual data to detect and correct errors without any retransmission of the data bits in error.
GEO: This describes a geosynchronous satellite angle with zero inclination so the satellite appears to hover (at an altitude of approximately 22,300 miles/35,786 kilometers) over one spot on the Earth’s equator. In reality, it’s a matter of matching the satellites movement to the earth’s rotation, so that–in relative terms–they match each other’s motion.
Gigahertz (GHz): One billion cycles per second. Signals operating above 3 GHz are known as microwaves; above 30 GHz they are known as millimeter waves. As one moves above millimeter waves, signals begin to take on the characteristics of light waves.
Intermediate Frequency (IF): Usually either 70 MHz or 140 MHz, IF modulation is used to provide a reliable, well understood intra-station transmission path for satellite signals.
Ka-band: The frequency range from 20 to 30 GHz. Now under development for many two-way broadband-by-satellite applications.
Klystron Tube Power Amplifier: A type of high power amplifier which uses a special klystron beam tube. A klystron tube typically covers the frequency of a single transponder; namely 40 MHz in the C-band and 80 MHz in the Ku-band.
Ku-band: The frequency range from 10.9 to 17 GHz. Increasingly used by communications satellites, it uses smaller dishes than those required for the C-band.
Low Noise Amplifier (LNA): This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port.
Low-Noise Block Downconverter (LNB): A combination Low Noise Amplifier and downconverter built into one device, which is attached to the feedhorn.
Megahertz (MHz): The frequency equal to one million Hz, or cycles per second.
Modulator: A device that modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel.
Mosquito: An annoying blood-sucker whose audio output is equivalent to a picowatt’s worth of RF signal. (See Picowatt.)
Multiplexing: Techniques that allow a number of simultaneous transmissions over a single circuit.
QPSK (Quadrature Phase Shift Keying): A digital modulation scheme used in transmission communications to allow increased sending capacity; twice as efficient as BPSK.
Picowatt: A unit of energy that is 0.0000000000001, or 10-12, watts in strength. (See Mosquito.)
Radio Frequency (RF): The actual signal sent out or received by the antenna.
Solid State Power Amplifier (SSPA): A solid state device that is lighter and often more reliable than conventional TWT amplifiers.
Subreflector: A device placed between a transmission feedhorn and the main dish surface. It allows the feedhorn to face upwards, thus putting its back to earth-generated thermal interference
Travelling Wave Tube (TWT) Amplifier: A tube-based broadband amplifier capable of covering all 24 transponders on a satellite.
Uplink: An earth station that transmits signals up to a satellite in Earth orbit.
Waveguide: A rectangular or elliptical tube used for transmitting microwave frequencies between the indoor electronics to an antenna on top of a tower/building; usually 2 GHz and up.
James Careless is a contributing writer to Via Satellite.